1. Field of the Invention
This invention is concerned with the use of changes in the light-transmission characteristics of optical fibers due to microbending with application to detecting intrusion of a protected area.
2. Discussion of the Prior Art
It is well known that when a coherent light beam is transmitted through a multimode optical fiber, certain characteristics of that light beam are altered when the fiber is disturbed by microbending. Because the optical properties of a fiber are profoundly altered by a mechanical disturbance of the fiber, in some applications, such as in the communication arts, great pains are taken to shield and protect the fiber. On the other hand, that very sensitivity to mechanical disturbance makes an optical fiber a fine candidate for use as an intruder detection sensor.
U.S. Pat. No. 4,297,684 to C. D. Butter teaches the concept of burying an optical fiber under a limited area to be protected. A coherent light beam is directed through a length of the fiber. The output light image is a speckled interference pattern that changes in appearance when the fiber is deformed by an intruder walking thereon. The pattern change indicates the fact that a disturbance has taken place.
In U.S. Pat. No. 4,488,040 to D. H. Howe, a strand of aramid cord is wrapped around an optical fiber. The aramid-wrapped fiber in encased in a sheath of flexible material such as a tetrafluorethylene fluorocarbon resin. When the sheath is squeezed, the aramid cord creates microbends in the fiber. The assembly is buried in the ground. A laser sends a beam of coherent light down the fiber from one end. At the other end, a detector measures the change in polarization of the emerging light beam when the fiber is disturbed. Detection of a change in polarization activates an alarm signal.
H. E. Solomon, in U.S. Pat. No. 4,903,339, teaches a method for detecting intrusion of a communications system. Here, the original data signal includes a synchronizing periodic waveform. The receiver generates an inverted synchronizing waveform that is nulled against the transmitted waveform. When the system is violated, power is extracted from the system so that the nulled condition is disrupted and an alarm signal is set off.
A somewhat similar system is taught by my U.S. Pat. No. 4,965,856 assigned to the assignee of this invention. Here, a reference signal is transmitted from a first location to a second location over an optical-fiber communications link. A replica of the reference signal is transmitted to the second location over a separate communications channel. At the second (receiver) location the phase shift between the reference signal and its replica is measured. An alarm is sounded when the phase shift departs significantly from a specified value.
All of the specimens of known art teach qualitative alarm systems that simply announce the fact that an intrusion has taken place but without making any sort of quantitative judgement of who or what caused the intrusion. It would seem wasteful of resources, for example, to call out the bloodhounds to pursue a stray baseball that happened to land on an intruder-detection array near a prison fence.
It is an object of this invention to provide a smart system for estimating the genus of an entity intruding into a secure area.